A frequent problem encountered when coating low-temperature substrates, such as wood, with coating powder is non-uniformity of coating in areas of the substrate which are difficult to coat, such as the edges and corners of kitchen cabinet doors. A frequently observed defect at edges, corners and other surface discontinuities is a rough, non-uniform finish. Surface moisture on wood provides a mechanism for the electrostatic coating of the otherwise non-conductive substrate, particularly when the substrate is grounded. The Faraday cage effect, the self limiting coating thickness and the ability of the powder to follow electrostatic field lines and wrap around edges, all typical electrostatic coating effects, are all found when wood is electrostatically coated at room temperature. Preheating of the wood to the powder fusing temperature defeats the self limiting mechanism so that thicknesses of 5 mils or more are achieved. Some improvement in Faraday penetration is also seen and the wrap around phenomenon is unchanged.
When a wooden substrate, e.g., medium density fiberboard or MDF, is machined into a complex part such as a cabinet door, a groove routed within the field of the door's front surface tends to exhibit problems of incomplete coverage that may be associated with the Faraday effect. Other machined areas of the door which seemingly should be easy to coat electrostatically, such as the outside edges which make up the perimeter of the door, also pose problems which make complete coverage very difficult. Because the thinned areas of the machined door surface tend to lose heat from the pre-heated door more rapidly than the areas of original thickness, the outside edges and corners become the most difficult to coat properly.
For example, the front side (or A side) of a cabinet door made from one piece of medium density fiberboard typically is routed along its perimeter to help give the visual appearance of a traditional "5 piece" natural wood cabinet door. The profile of this perimeter routing (the classical Ogee, for example) depends upon the selection of the proper router bit but it always results in a declining step shape, a portion of which is thinner than the original edge. Coating these thinner portions first from the front side before much heat is lost would be the logical approach but the result is unsatisfactory.